Low-Temperature Combustion Characteristics of a n-Butanol

Article pubs.acs.org/EF

Low-Temperature Combustion Characteristics of a n‑Butanol/ Isooctane HCCI Engine Bang-Quan He,*,† Jie Yuan,† Mao-Bin Liu,† and Hua Zhao†,‡ †

State Key Laboratory of Engines, Tianjin University, 92 Weijing Road, Tianjin 300072, PR China Centre for Advanced Powertrain and Fuels, Brunel University, Kingston Lane, London UB8 3PH, U.K.

‡

ABSTRACT: Biobutanol (i.e., n-butanol) can be used as a substitute for spark ignition and homogeneous charge compression ignition (HCCI) engines. Its low-temperature reactions will determine the autoignition process and will have a significant effect on the heat release rate in the HCCI engines. In order to understand the temporal evolution of such reactions, the formation and concentration of intermediates formed in the oxidation of n-butanol was studied by means of a fast gas sampling probe in a single-cylinder four-stroke HCCI engine. The species in the sampled gas were identified through gas chromatography. Isooctane was used as a reference fuel to find the effect of fuel structure on the low-temperature reactions. The dominant reaction routes related to the formation of intermediate products and autoignition for n-butanol and isooctane were investigated through sensitivity analysis. The results show that for the air−fuel mixture diluted by residual gases in the cylinder, n-butanol autoignites more easily than isooctane at low temperature. For n-butanol and isooctane, the concentration of each species left in the residual gases during the negative valve overlap period is much lower than the peak concentration of its counterpart during the compression stroke. The formation of a large amount of isobutene in the cylinder before autoignition for isooctane is the cause for its late autoignition. The dominant reaction routes affecting the formation of species and autoignition timing are dependent on fuel structure. (MFB) toward top dead center (TDC).3 However, n-butanol was found more prone to knocking combustion than gasoline.2,4 Furthermore, hydrocarbon (HC) and carbon monoxide (CO) emissions were affected by butanol volume fraction in the butanol−gasoline blend.5 Compared to gasoline, n-butanol blends could reduce CO and HC emissions.4,6 Pure n-butanol can decrease NOx emissions and particle number concentration.6 In a single-cylinder port-fuel injection SI engine fuelled with butanol/isooctane blends, Broustail et al.7 found that HC and NOx emissions can be significantly decreased. Meanwhile, ethylene and benzene emissions decrease while formaldehyde emissions increase. Homogeneous charge compression ignition (HCCI) is different from conventional SI and compression ignition (CI) operations. HCCI combustion is characterized with spontaneous autoignition and heat release reactions in a premixed fuel and air mixture. Compared to conventional SI operations at part loads,8,9 significant improvement in fuel economy can be obtained due to rapid heat release processes and lowtemperature combustion as well as the lower pumping loss associated with throttled SI operation. There are many approaches to achieve HCCI combustion in engines.10−12 From the standpoint of practical application, trapping residuals by the negative valve overlap (NVO) strategy from a previous cycle is one of the most effective ways to initiate HCCI combustion in passenger car gasoline engines.13 In this context, the exhaust valves are closed early in the exhaust stroke so that a substantial amount of hot residual gases is retained in the

1. INTRODUCTION Concerns over future petroleum supply demands and environmental damage from burning fossil fuels have led to substantial interest in the production and usage of biomass-derived fuels, due to the improved energy security and lower lifecycle emissions of carbon dioxide (CO2) by the substitution of conventional fuels with biofuels. There are a multitude of biofuels. The first-generation biofuels including ethanol and biodiesel are produced mainly from agricultural crops for food and animal feed purpose. The second generation biofuels can be manufactured from various types of biomass including wood, vegetable waste, and more generally lignocellulosic materials, offering an even more favorable well-to-wheel CO2 balance. Therefore, the second generation biofuels are superior to the first generation in terms of energy balances, the reduction of greenhouse gas emissions, and the competition for food. Biobutanol (i.e., n-butanol) is one of the second-generation biofuels that is well suited for internal combustion engines due to its similarity in physical properties to gasoline. In comparison with ethanol, n-butanol has higher energy density. It is less corrosive and is much less prone to water contamination.1 Hence, n-butanol is more compatible with existing engines when blended with conventional fuels and it can be used with the existing fuel supply infrastructure. In addition, it has lower vapor pressure for better startability. Therefore, n-butanol is a potential replacement for ethanol as an oxygenate or fuel.2 Butanol has been investigated on conventional spark ignition (SI) engines in the past. Alasfour3 found that the reduction of thermal efficiency was about 4.5% when the engine was fuelled with a 30% isobutyl alcohol−gasoline blend at stoichiometry as compared to gasoline. n-Butanol can shorten early burn duration and advance the location of 50% mass fraction burned © 2014 American Chemical Society

Received: March 3, 2014 Revised: April 27, 2014 Published: May 12, 2014 4183

dx.doi.org/10.1021/ef500508h | Energy Fuels 2014, 28, 4183−4192

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can also have significant impact on the low-temperature oxidation reactions. In order to understand the critical roles of species formed in the cylinder on the autoignition timing of HCCI engines, the time histories of the intermediates’ formation and consumption in the cylinder in the oxidation of fuel were investigated through rapid gas sampling from the cylinder in this study. To find the effect of molecular structure on HCCI combustion, isooctane, a surrogate fuel for gasoline, was used as a reference fuel. The motivation of this work is to extend the knowledge in oxidation and combustion kinetics of n-butanol and isooctane in an HCCI engine.

cylinder in order to raise the charge temperature of the next cycle and achieve HCCI combustion. Therefore, the load of the HCCI engines is controlled by the residual gas fraction in the cylinder. However, the autoignition timing of HCCI engines is very difficult to control over the entire engine speed-load regions, because it is completely controlled by chemical kinetics. Therefore, it is influenced by fuel composition, the equivalence ratio, and the thermodynamic state of the mixture in the cylinder. Furthermore, a high heat release rate leads to significant noise and potential engine damage. Hence, the high load of HCCI engines is limited either by knock at low speed or by misfire at high speed due to insufficient heat energy from a small amount of residual gas retained in the cylinder. On the other hand, the low load of HCCI engines is limited by misfire, because the temperature of residual gases trapped is too low to achieve HCCI combustion.14,15 Consequently, HCCI-SI mode transition is required for the application of HCCI combustion to the engines of passenger cars. Furthermore, high HC and CO emissions of some HCCI engines need to be solved through the oxidation catalysts with low light-off temperature. n-Butanol can be used on HCCI engines as a fuel or a gasoline substitute. However, to our knowledge, only a few studies were conducted on the HCCI engines fuelled with butanol gasoline blends.16−18 In addition, much of the previous research focused on the kinetic reactions of butanol in atypical in-cylinder conditions.19−22 In a study carried out in a shock tube at the temperature range from approximately 1200K to 1800K and the pressure range of 0.1−0.4 MPa, Moss et al.23 found that dehydration, unimolecular decomposition, and H atom abstraction are the relatively important consumption reactions in the oxidation of four butanol isomers. In the counterflow twin-flame configuration at atmospheric pressure and the temperature of 343 K for a wide range of equivalence ratios, Veloo et al.24 found that the propagation of n-butanol/ air flames is faster than that of both sec-butanol/air and isobutanol/air flames, whereas the propagation of tert-butanol/air flames is slower compared to the other three isomers. Hansen et al.25 developed a predictive, comprehensive reaction mechanism for the high-temperature oxidation chemistry of n-butanol and found that the consumption of n-butanol is mainly from the H atom abstraction with H, O, and OH radicals. In a jet-stirred reactor performed in the temperature range of 770−1190K, at 1 MPa, and stoichiometric condition, Dagaut and Togbé26 found that with the increase of initial fraction of alcohol in isooctane, ethanol or butanol increases the importance of its reaction routes for the formation of the products. In a rapid compression machine performed over a temperature range of 920−1040 K and a pressure range of 0.286−0.335 MPa for stoichiometric n-butanol/O2 mixture, Karwat et al.27 investigated the mole fraction time histories of the formation of species and found that the reactions were important for the prediction of ignition delay time and the formation of intermediate species. From previous studies, authors17,18 found that the onset of autoignition timing advances with increasing n-butanol volume fraction during the HCCI operation, which has an adverse effect on the engine thermal efficiency. The oxidation processes leading to the autoignition in the cylinder are dependent on fuel structure, temperature, the amount of fuel burned, local air/fuel ratio, and engine speed. The differences in the molecular structure change the reaction path and the amount of intermediates from fuel to final products. In addition, the temporal variation in the mixture composition and temperature

2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. A single-cylinder fourstroke gasoline engine with variable valve timing (VVT) and variable valve lift (VVL) on both intake and exhaust valves was used. Its bore and stroke were 86 mm × 86 mm, and its compression ratio was 10.66. Fuel was injected into the intake ports by a four-hole injector at a gauge pressure of 0.29 MPa. A MAX 213 positive-displacement piston-type flow meter with an accuracy of ±0.2% was used to measure fuel flow rate. With the help of a linear oxygen sensor, the relative air/fuel ratio was measured through an ETAS lambda meter with an accuracy of ±1.5%. Through a Kistler 6125B piezoelectric transducer coupled with a Kistler 5011B charge amplifier and a data acquisition system, in-cylinder pressure was recorded, and its ensemble average over 100 consecutive cycles was used to analyze the combustion processes of the HCCI engine in this study. To investigate the time histories of intermediates with crank angle when n-butanol and isooctane were burned in the HCCI engine, gases in the cylinder were sampled in the period of NVO and compression stroke and at the initial stage of autoignition. The gas sampling system is shown in Figure 1. It

Figure 1. Scheme of gas sampling system.

consisted of a personal computer, a rapid response solenoid valve, a flow meter, a sampling bag, and two valves. The solenoid valve was driven by an electronic control unit, and its opening timing and duration in a sample was controlled by the computer. The number of samples was decided by the amount of gas flow passing through the meter. To diminish the effect of gas sample on the cycle-by-cycle combustion variation, the number of samples in each operating condition was over 5000 since only 0.1−0.2% of the gases in the cylinder was sampled in a cycle. To avoid the flame extinguishing effect of combustion chamber walls on species measured, the distance between the tip of the probe and the walls was 5 mm. To quantify the species interested in the sample gases, a SP-3420 gas 4184


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chromatograph (GC) system with a resolution of 8 × 10−12 g/s (n-C16) was used. 2.2. Experimental Procedures. With a 30 kW AC electric dynamometer, the engine was motored and started in the SI mode with a stoichiometric mixture at 1500 rpm. Then, it was switched into the HCCI mode through altering the intake and exhaust valve timings to trap sufficient hot exhaust gases. During the HCCI operation, the spark ignition was turned off and the throttle remained wide open. During all the experiments, the temperatures of coolant and oil were maintained at 80 ± 1 °C and 55 ± 1 °C, respectively. n-Butanol and isooctane were used in the experiments. The purities of n-butanol and isooctane were 99.7% and 99.5% by volume, respectively. The research octane number of n-butanol was measured according to the GB 5487-1995 test method, and it was greater than 100. The experiment conditions at the engine speed of 1500 rpm are shown in Table 1.

during the NVO period, the sample duration was longer, because there was almost no chemical reaction in the cylinder. Prior to the start of sampling, valve 1 was opened while valve 2 was closed. In this case, the sampled gas was used to clean out the residual gas in the sampling system. When sampling took place, valve 2 was opened while valve 1 was closed. As a result, the sampled gases from the cylinder flowed into the sampling bag due to the pressure differential between the cylinder and the bag. The sampled gases underwent a rapid expansion after the sampling valve, resulting in the sudden decrease in temperature in the sample system. Hence, the oxidation of the species in the sampled gases was quickly quenched, and the concentration of the species in the gases could be analyzed through the SP-3420 GC system, in which a flame ionization detector (FID) was maintained at 230 °C, a HP-AL/S (50 m × 0.53 mm × 15.0 μm) capillary column was kept at 100 °C, and injection temperature was 200 °C. High-purity reference chemicals including methane (CH4), acetylene (C2H2), ethylene (C2H4), ethane (C2H6), propene (C3H6), 1-butene (1-C4H8), and isobutene (iC4H8) were characterized in the GC system, and their chromatograms were used to calibrate the absolute concentrations of the sample gases. The quoted purities of the reference gases were 99.99%. When analyzing the concentrations of the species in the sampled gases, 0.15 mL of sample gases and 10 μL of reference gases were introduced into the sample loop, respectively. Accordingly, the species in the sample gas can be identified through the comparison of their retention times between the sampled gas and reference gases. Figure 2 shows a GC-FID chromatogram when the HCCI engine operated with n-butanol fuel. The peaks of CH4, C2H6, C2H4, C3H6, C2H2, and 1-C4H8 can be identified in the figure.

Table 1. Maximum Intake and Exhaust Valve Lifts and Their Timings fuel

EVP /°CA BTDC

EL/mm

IVP /°CA ATDC

IL/mm

n-butanol isooctane

163 163

0.60 0.75

128 128

2.5 2.0

In Table 1, EL and EVP are the maximum exhaust valve lift and its corresponding timing, respectively. IL and IVP are the maximum intake valve lift and its corresponding timing, respectively. BTDC and ATDC denote the crank angles before and after top dead center of exhaust stroke, respectively. In order to achieve stable HCCI operation, isooctane fuel was operated with higher EL and lower IL than those for n-butanol. In these conditions, the residual gas fraction in the cylinder was 65% for n-butanol and was 65.4% for isooctane. The coefficient of variation of indicated mean effective pressure (IMEP) of the HCCI engine was below 5%. The sampling timing and duration are shown in Table 2. Table 2. Sample Timings and Duration sample pulse timing /°CA BTDC

sample pulse width/ms

sample duration /°CA

actual sample timing /°CA BTDC

375 60 45 30 20 10 0

3 2 2 1.8 1.8 1.6 1.6

27 18 18 16.2 16.2 14.4 14.4

361.5 51 36 21 12.8 2.8 −7.2

Figure 2. GC-FID chromatogram results of a sample gas with nbutanol.

CH4 concentration in the sample gases could be obtained through the peak area ratio of CH4 in the sample gas to that of CH4 reference gas. The concentrations of the others in the sample gases can be obtained on the basis of their relative mass calibration factors and CH4 concentration according to ref 28.

In Table 2, sample duration was calculated by the sample pulse width at the engine speed of 1500 rpm. The actual sample timing was represented as the crank angle corresponding to the midpoint of the sampling duration. It took about 0.5 ms to fully open the sample valve after sending a trigger signal. The sampling duration was changed according to the crank angle to be sampled. It was longer for the samples early in the compression stroke, due to low in-cylinder pressure and low oxidation rate of fuel. However, it was shorter for the samples taken late in the compression stroke because of high oxidation rate of fuel and high in-cylinder pressure. For the samples

3. RESULTS AND DISCUSSION 3.1. Modeling of Chemical Kinetics of n-Butanol and Isooctane in HCCI Combustion. To understand the mechanisms to generate the intermediate products in the cylinder of the HCCI engine, one-dimensional simulation was 4185


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Figure 3. Comparison of experimental and simulated results for n-butnaol (a) and isooctane (b).

3.2. Comparison of In-Cylinder Temperature and Heat Release Rate for n-Butanol and Isooctane. In-cylinder temperature (T) can be used to evaluate the oxidation processes of fuel in the HCCI engine. It is calculated as follows.

conducted on the basis of the Chemkin software package coupled with the detailed chemical reaction mechanisms of nbutanol and isooctane developed by the Lawrence Livermore National Laboratory.29 The chemical reaction mechanism for nbutanol was validated in shock tube, rapid compression machine, and jet stirred reactor in the pressure range of 0.1− 8 MPa, in the temperature range of 720−1700 K, and the equivalence ratio (Φ) range of 0.6−1.7. The validity ranges for isooctane in shock tube and rapid compression machine were in the pressure domain between 0.3 and 5 MPa, temperature between 650 and 1200 K, and Φ between 0.3 and 1.0. Based on the experimental results, the residual gases in the cylinder at the TDC were supposed to be composed of CO2, water (H2O), nitrogen (N2), CH4, C2H6, C2H4, C3H6, C2H2, and butane when a numerical simulation was conducted. Their mole fractions in the residual gases at the TDC were obtained in an iterative manner. The in-cylinder temperature at the intake valve closing (IVC) timing was calculated on the basis of the in-cylinder pressures measured. The simulation was performed from the IVC timing to the exhaust valve opening (EVO) timing, in which compression, combustion, and expansion strokes occurred in the cylinder. Figure 3 shows the comparison of in-cylinder pressure and heat release rate between experimental and modeling results. According to ref 30, the time when 20% fuel was consumed was chosen in the simulation as autoignition timing, which corresponds to the crank angle where 10% fuel has been burned in the cylinder in the experiment.31 It can be seen that the pressure curves of experiment and simulation are very close at the initial stage of combustion for nbutanol and isooctane. For n-butanol, its autoignition timing is 4.55 °CA BTDC in the experiment and 4.27 °CA BTDC in the simulation. For isooctane, its autoignition timing is 2.64 °CA ATDC in the experiment and 2.82 °CA ATDC in the simulation. However, the peak rate of heat release in the simulation is much higher than that in the experiment for both fuels. This could be the result of the assumption of homogeneous mixture and temperature distributions in the cylinder, which would undergo simultaneous autoignition combustion and hence rapid heat release. In the actual engine, there will always be slight spatial variations in mixture and temperature uniformity. However, the modeling study focused on the early stage of the fuel oxidation reactions will help to elucidate the dominant chemical reaction routes involved and to understand the experimental results relating to the intermediates formation of n-butanol and isooctane during low temperature combustion.

pV (1) nR where p is in-cylinder pressure, V is the cylinder volume, n is the total mole number (including air, fuel, and residual gases), and R is the universal gas constant. Figure 4 shows in-cylinder temperature and heat release rate when n-butanol and isooctane were used in the HCCI engine in T=

Figure 4. Experimental comparison of in-cylinder temperature and heat release rate for n-butanol and isooctane.

the conditions shown in Table 1. It can be seen that although isooctane is subject to slightly higher effective compression ratio due to advanced IVC timing than n-butanol, its autoignition occurs later and combustion duration is longer than n-butanol. In addition, n-butanol produces a higher peak of heat release rate. However, it is noted that isooctane is subject to higher temperature at the same crank angle during the compression stroke than n-butanol. This indicates that nbutanol is more prone to autoignition than isooctane under similar conditions. n-Butanol is a straight-chain alcohol containing the CH3−CH2−CH2−CH2− group, which readily forms C−C−C−O−O−H32 and then the OH radical by breaking the O−O bond, whereas isooctane does not easily undergo internal H atom abstraction bound to primary sites.33,34 The presence of tertiary and quaternary C atoms in isooctane also suppresses autoignition.35 Because n-butanol is predominantly consumed by hydroxyl (OH) radicals,36,37 it 4186


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Figure 5. Comparison of C2H4 concentration in the cylinder for experiment (a) and simulation (b).

By examination of Figure 4 and Figure 5a, it is noted that isooctane produces less C2H4 at higher in-cylinder temperature than n-butanol, due to different reaction routes when they are oxidized at low temperature. Broustail et al.7 found a similar trend in a port-fuel injection SI engine. A comparison between experimental and modeling results shows that the trends of C2H4 concentration profiles with crank angle are similar for the two fuels. But the crank angle where C2H4 concentration rapidly increases occurs much early in the experiment. It is likely caused by the active radicals left from the previous cycle promoting the oxidation of fuel at low temperature. Furthermore, the peak of C2H4 species in the cylinder sustains longer in the experiment, which may be attributed to the slight inhomgeneous distribution of the mixture and temperature in the cylinder. The simulation results of the other species follow the same trends, although there are discrepancies between the absolute values. Hence, the simulation results can be used to delineate the dominant chemical reaction routes of the species formation in this study. Figure 6 shows C3H6 concentration profiles as a function of crank angle when n-butanol and isooctane were burned in the

indicates that OH radicals can be formed much more easily from the oxidation reactions of n-butanol than isooctane at relative low temperature, because of their different reaction routes.5 The retarded autoignition is also related to the formation of large amount of iC4H8 from the oxidation of isooctane, as shown in Figure 8, because the formation of iC4H8 results in the reduction of radical levels.38 3.3. Comparison of In-Cylinder Intermediates Produced by n-Butanol and Isooctane. Figure 5 shows C2H4 concentration profiles as a function of crank angle. It can be seen that the experimental and simulated results follow the same trends. However, the C2H4 concentration is overpredicted in the simulation. One reason is that simulation results are based on a one-zone combustion model, in which in-cylinder temperature and compositions of the mixture in the cylinder are supposed to be homogeneous, causing a rapid heat release rate shown in Figure 3, as compared with experimental conditions. Another reason is that the C2H4 concentration at a crank angle represents an average value of the species during the gas sampling duration in the experiment, which decreases the magnitude of C2H4 concentration, consistent with ref 39. Furthermore, C2H4 is found at −361.5 °CA ATDC and during the intake stroke in the experiment because of the residual species left from the previous cycle. As shown in Figure 5a, C2H4 production from n-butanol starts to rapidly increase around −36 °CA ATDC where incylinder temperature is about 880 K, as shown in Figure 4, and heat release appears. C2H4 concentration increases and reaches its maximum at about −21 °CA ATDC where in-cylinder temperature is about 970 K (Figure 4). Hereafter, C2H4 concentration dramatically decreases accompanied by a significant increase in the charge temperature. When in-cylinder temperature reaches about 1020 K at −12.8 °CA ATDC (Figure 4), the chain-branching reaction H2O2(+ M)OH + OH (+ M) takes place, in which M is any third body involved in the reaction. OH will consume C2H4, and hence C2H4 reaches its minimum. In the case of isooctane, C 2 H 4 concentration starts to rise at −36 °CA ATDC, where the incylinder temperature is about 900 K (Figure 4), and jumps at −21 °CA ATDC, where the in-cylinder temperature is about 990 K. C2H4 reaches its maximum concentration at −12.8 °CA ATDC and about 1020 K (Figure 4). Then, it decreases with crank angle and reaches its minimum concentration at 8.2 °CA ATDC, a crank angle later than the autoignition timing of 2.64 °CA ATDC in this condition.

Figure 6. Experimental comparison of C3H6 concentration in the cylinder for n-butanol and isooctane.

HCCI engine. It can be seen that the maximum C3H6 concentration occurs at an earlier crank angle but at a lower value with n-butanol than isooctane. This is because the formation of C3H6 is mainly from the reaction iC4H9 and iC3H7, which are more readily formed from low-temperature reactions of isooctane, as shown in Figure 12. Figure 7 shows C2H2 concentration profiles as a function of crank angle when n-butanol and isooctane were burned. The 4187


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Figure 9. Experimental comparison of CH4 concentration in the cylinder for n-butanol and isooctane.

change in C2H2 concentrations with crank angle for the two fuels follows the same trend. It can be seen that the maximum concentration of C2H2 is less than those of C2H4 and C3H6. C2H2 is formed through the reactions consuming C2H4 and C3H6, as shown in Figures 11 and 12, because of lower bond energy of C2H4 and C3H6 relative to that of C2H2. Figure 8 shows C4H8 concentration profiles as a function of crank angle when n-butanol and isooctane were burned in the

concentration of CH4 appears at an earlier crank angle for nbutanol than that of isooctane. On the other hand, CH4 concentration from n-butanol is lower than that of isooctane. For n-butanol, CH4 can be produced by reactions of CH3 with n-butanol (nC4H9OH), HO2, and CH2O, as shown in Figure 11. However, for isooctane, CH4 is generated by the reaction of CH3 with isooctane (iC8H18), CH2O, and HO2, as shown in Figure 12. Because isooctane is a branched alkane with three methyl groups in its chain, more CH3 radicals can be formed during the oxidation of isooctane.26 Consequently, much CH4 is produced in the cylinder. Figure 10 shows C2H6 concentration profiles as a function of crank angle when n-butanol and isooctane were burned. The


HCCI engine. Because the molecular structures of n-butanol and isooctane are quite different, C4H8 has different structures for the two fuels. For n-butanol, β fission of bC4H8OH, a radical formed by H abstraction from the β carbon of n-butanol, produces 1-C4H8, whereas isooctane produces iC4H8. Yang et al.40 found that 1-butene is the predominant C4H8 in the premixed, fuel-rich n-butanol−oxygen flames at an initial equivalence ratio of 1.71 and a pressure of 4 × 10−3 MPa. However, the consumption of n-butanol through H abstraction from the β carbon is less than that from α carbon as shown in Figure 11. On the contrary, iC4H8 can be produced through the routes of tC4H9 (a tertiary butyl radical), yC7H15 (a 2,4dimethyl-propan-2-yl radical), aC8H17 (a radical formed by H abstraction from the α carbon of isooctane) and cC8H17 (a radical formed by H abstraction from the γ carbon of isooctane) shown in Figure 12. As a consequence, the concentration of iC4H8 is higher than that of 1-C4H8. Figure 9 shows CH4 concentration profiles as a function of crank angle when n-butanol and isooctane were burned. It can be seen that CH4 concentration reaches its maximum before autoignition timing for the two fuels. But the peak


main route to form C2H6 is CH3 + CH3, as shown in Figures 11 and 12. Because there are more reaction routes leading to the production of CH3 from isooctane, more C2H6 is formed from isooctane. In addition, it is noted that in the case of n-butanol, C2H6 has the lowest peak concentration among all the species identified, due to the longer and smaller number of reaction routes to form CH3. Similarly, C2H4 is more abundant than C3H6 because of more reaction routes to generate C2H4, as shown in Figure 11. However, in the case of isooctane, the opposite is the case, because there are more reaction routes to generate C3H6, as shown in Figure 12. 3.4. Reaction Kinetic Analysis of n-Butanol and Isooctane. 3.4.1. Oxidation Reaction Paths at Low Temperature. HCCI combustion is kinetically controlled. To identify the main reaction paths of the formation of intermediate products discussed above during the oxidation of n-butanol and 4188


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Figure 11. Reaction path diagram of the main decomposition routes to form the notable intermediates from n-butanol at low temperature (PIVC = 0.1 MPa, TIVC = 535 K, Φ = 1.0).

Figure 12. Reaction path diagram of the main decomposition routes to form the notable intermediates from isooctane at low temperature (PIVC = 0.1 MPa, TIVC = 558 K, Φ = 1.0).

isooctane at low temperature, the production and consumption rates of the low temperature reactions were determined. Figure 11 shows the reaction path diagram of the main decomposition routes to produce C2H4, C3H6, C2H2, 1-C4H8, CH4, and C2H6 from n-butanol consumption at the initial conditions of PIVC = 0.1 MPa, TIVC = 535 K, and Φ = 1.0. Here, PIVC and TIVC are the in-cylinder pressure and temperature at IVC timing, respectively. TIVC was calculated by eq 1. In the figure, the percentages of reactants consumed to form an intermediate are shown in bold italic text, and the percentage in each reaction path to form a product is listed in plain text. The numbers are obtained by integrating the amount of production

or consumption of each species in each reaction up to the time of 20% fuel consumption and normalizing each reaction by the total generated or consumed of each species in this period. In the figure, aC4H8OH, bC4H8OH, cC4H8OH, dC4H8OH are the radicals formed via H abstraction from the α, β, γ, and δ carbon according to their distance from the hydroxyl moiety in nbutanol, respectively. pC4H9O is the radical via H abstraction from OH group of n-butanol. C4H71−1 is a radical formed via H abstraction of 1-C4H8. It is found that H abstraction via OH is the key decomposition pathway for n-butanol in this condition. The consumption percentage via OH from the α, γ, δ, β carbon and 4189


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for 14.2% at low temperature, which is higher than that of nbutanol shown in Figure 11. The contribution rates to produce iC4H8 via the reaction paths starting from aC8H17, cC8H17, and dC8H17 account for 25.4%, 21.7%, and 6.6%, respectively. The contribution rate to the formation of iC4H8 via the fission of yC7H15 is 15.6%. As shown in Figure 8, iC4H8 is the main intermediate product at low temperature, which is consistent with the results found by Curran et al.34 For the formation of C3H6, the reaction of iC4H9 via CH3 scission contributes to 46.8%. Because aC8H17 is the dominant reaction of H abstraction, C3H6 is higher in the oxidation of isooctane than that from n-butanol shown in Figure 6. Besides, the reaction paths from iC3H7 and dC8H17 also contribute to 18.0% and 7.8% of C3H6, which is different from the case of nbutanol. As for C2H4, the reactions C3H6 + HC2H4 + CH3 and C2H5 + O2C2H4 + HO2 contribute to 51.2% and 31.8% of C2H4, respectively, whereas the reaction C2H5(+ M)C2H4 + H(+ M) contributes to 12.9% of C2H4. It is obvious that most C2H4 is converted from C3H6. Accordingly, C2H4 concentration is lower than C3H6. For the formation of C2H2, the reaction C3H4-p + HC2H2 + CH3 contributes to 46%. Because C3H4-p is a direct cleaved product of isooctane, it is the dominant route to form C2H2. In addition, the reaction routes from C3H6 and C2H4 contribute to 34.1% and 10.5% of C2H2, respectively. The reaction paths to form CH4 and C2H6 from isooctane are the same as those from n-butanol. The quantity of them is determined by CH3 radical yield during low temperature oxidation. For isooctane, CH3 radicals are easily produced when iC8H18, iC4H9, bC8H17, and neoC5H11 are decomposed. The reactions between CH3 and iC8H18, HO2, and CH2O via H abstraction contribute to 31.8%, 22.4%, and 23.3% of CH4, respectively. On the other hand, the recombination reaction CH3 + CH3(+ M)C2H6(+ M) produces 99.8% of C2H6. Hence, the amount of CH4 and C2H6 is higher for isooctane than that for n-butanol, as shown in Figures 9 and 10. 3.4.2. Sensitivity Analysis at Low Temperature. Sensitivity analysis was conducted to ascertain the most sensitive reactions in the oxidation of n-butanol and isooctane at the temperatures shown in Figures 11 and 12, respectively. The percent sensitivity is defined as

O−H of n-butanol is 39.3%, 14.7%, 14.2%, 9.3%, and 1.7%, respectively. It means that the primary consumption route is the H abstraction from the α carbon of n-butanol, in that the magnitude of the bond energy between carbon and H in nbutanol increases in the order of Cα-H < Cγ-H < Cβ-H < Cδ-H < O−H.30 Furthermore, the reaction of n-butanol with HO2 consumes 17.6% of n-butanol. As shown in Figure 11, C2H4 is mainly formed through the reaction routes of C−C bond scission of aC4H8OH and dC4H8OH. The reactions C2H5 + O2C2H4 + HO2 and C2H5(+ M)C2H4 + H(+ M) contribute to 47.4% and 6.5% of C 2 H 4 , respectively, whereas CH 3 scission from nC 3 H 7 contributes to 21.3% of C2H4. The contribution rate of the reaction of β scission from dC4H8OH to C2H4 is 12.1%, and the fission of pC2H4OH (CH2−CH2−OH) contributes to 11.1% of C2H4. However, the decomposition of n-butanol only accounts for 0.3% of C2H4. For the formation of C3H6 and 1-C4H8, β scission of cC4H8OH and bC4H8OH are the primary routes to generate them, respectively, in that the bond energies of Cγ-H and Cβ-H are higher than that of Cα-H, and the consumption of n-butanol through β scission of cC4H8OH and bC4H8OH is less than that of aC4H8OH. As a result, the concentrations of C3H6 and 1C4H8 are less than that of C2H4, which was verified in the experimental results. For the formation of C3H6, the contribution rate of C−C bond β scission of cC4H8OH to C3H6 is 97.0%. The reaction nC3H7 + O2C3H6 + HO2 is a minor contributor to the formation of C3H6 not shown in this figure. The decomposition of bC4H8OH via C−O bond β scission accounts for 98.5% of 1C4H8, whereas the reaction nC4H9OH(+ M)1-C4H8 + H2O(+ M) is a minor contributor to the formation of 1-C4H8. Moreover, the reaction routes starting from 1-C4H8, C3H6, and C2H4 contribute to 18.4%, 76.9%, and 4.1% of C2H2, respectively. Therefore, the formation time of C2H2 is later than those of 1-C4H8, C3H6, and C2H4, which was confirmed in Figures 5−8. In addition, the reactions of CH3 radicals with nC4H9OH, HO2 and CH2O via external H atom abstraction form CH4 and contribute to 39.9%, 46.1%, and 6.8% of CH4, respectively. For C2H6, the recombination reaction CH3 + CH3C2H6 is the main contributor, accounting for 63.2%, whereas the reaction C2H5 + nC4H9OH and C2H5 + HO2 also contribute to the formation of C2H6. Figure 12 shows the reaction path diagram of the main decomposition routes to form C2H4, C3H6, C2H2, iC4H8, CH4, and C2H6 from iC8H18 consumption at the initial conditions of PIVC = 0.1 MPa, TIVC = 558 K, and Φ = 1.0. In the figure, aC8H17, bC8H17, cC8H17 and dC8H17 are the radicals formed by H abstraction from the a, b, c, and d carbon of isooctane, respectively. yC7H14 is a 2,4-dimethyl-pent-2-ene radical. C3H4p is propadiene. It can be seen that the consumption of isooctane is favored by the reactions with OH radicals. The reaction paths to form the same species above are different from those of n-butanol consumption, due to their different molecular structures. Although some reaction routes for the formation of C2H4, C3H6, C2H2, CH4, and C2H6 are the same, their contribution rates to the formation of these species are quite different. Although H abstraction from isooctane is more difficult than that from n-butanol, it is still the dominant reaction for the consumption of isooctane. However, the decomposition of isooctane via the reaction iC8H18CH3 + yC7H15 accounts

sensitivity =

t(2k i) − t(k i) × 100% t (k i )

(2)

where ki is the rate constant of reaction i, t(ki) is the ignition delay time when the rate constant of reaction i is ki, and t(2ki) is the ignition delay time when the rate constant of a particular elementary reaction step i is multiplied by a factor of 2. Accordingly, the reaction with a positive percentage change will reduce the overall reactant rate of the system and increase ignition delay time. Vice versa, the reaction with a negative sensitivity coefficient will accelerate the overall reactivity of the system and decrease ignition delay time. Figure 13 shows the 12 most sensitive reactions in the autoignition process of n-butanol. It can be found that H abstraction in the α carbon of n-butanol is the dominant reaction path affecting n-butanol autoignition at low temperature. Furthermore, H abstraction from the α carbon of nbutanol is more sensitive to temperature than that from the β, γ, and δ carbon of n-butanol because of low bond energy of CαH affected by the OH group. Accordingly, nC4H9OH + HO2 = 4190


Energy & Fuels

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5. CONCLUSIONS The time histories of the intermediates formed in the cylinder were investigated in a single-cylinder four-stroke HCCI engine fuelled with n-butanol and isooctane. The dominant reaction routes to form the intermediates at low temperature were identified through sensitivity analysis. The following conclusions can be reached: (1) n-Butanol autoignites more easily and produces a higher rate of heat release than isooctane in a HCCI engine with the mixtures diluted by residual gases. (2) The earlier formation and consumption of ethylene and acetylene at low temperature condition is responsible for the more advanced autoignition timing of n-butanol than isooctane. In addition, the formation of a large amount of iC4H8 in the cylinder retards autoignition of isooctane in the HCCI engine. (3) During the negative valve overlap period, the concentration of the intermediates left in the residual gases from a previous cycle is dependent on the fuel used. For isooctane, its species concentration is higher than that for the case of n-butanol in most cases. However, the concentration of the species in the residual gases is much lower than the peak value of its counterpart in the oxidation of fuel during the compression stroke. (4) The dominant reaction routes and the contribution rate to the formation of species in the oxidation of n-butanol and isooctane are affected by their molecular structures. C4H8 with different structures are formed by n-butanol and isooctane. (5) The reactions with HO2 and OH have a great effect on the autoignition of n-butanol and iso-ocatne. For nbutanol, the most sensitive reaction to autoignition is C4H9OH + HO2 = aC4H8OH + H2O2, whereas for isooctane, the most sensitive reaction to its autoignition is iC8H18(+ M)CH3 + YC7H15(+ M).

Figure 13. Sensitivity analysis on the ignition of n-butanol.

aC4H8OH + H2O2 is the dominant reaction influencing autoignition of n-butanol. In addition, the reactions with HO2 and H2O2 have greater impact on autoignition. Among them, H2O2 + (M)OH + OH + (M) is a dominant reaction to promote autoignition while the reactions HO2 + OHH2O + O2 and HO2 + HO2H2O2 + O2 consume other free radicals and hence delay ignition. Karwat et al.17 also found the same results in a rapid compression machine operating in the temperature range of 920−1040K and in the pressure range of 0.286−0.335 MPa. As shown in Figure 13, the H abstraction of the α and γ carbon of n-butanol, a main consumption route, advances autoignition, whereas the H abstraction from the β and δ carbon and O−H delays autoignition. Also, the formation of aC4H8OH and cC4H8OH through β scission of n-butanol, followed by the production of C2H4, CH2O, and CH3CHO,21 promotes autoignition. Figure 14 shows 12 reactions most sensitive to the autoignition of iso-ocatne at low temperature. It can be seen

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-22-27383362. Tel.: +8622-27406842 ext. 8011. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support by the National Natural Science Foundation of China (NSFC) through the Project Grant No. 51076113 and the Ministry of Science and Technology (MOST) through the 973 Project Grant No. 2013CB228403.

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Figure 14. Sensitivity analysis on the ignition of isooctane.

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that H abstraction of isooctane is not sensitive to ignition delay, which is different from that of n-butanol. However, the reactions with HO2 and OH have inhibiting effect on the autoignition of isooctane, due to higher bond energy of C−H in isooctane. The reaction iC8H18(+ M)CH3 + YC7H15(+ M) has promoting effect on the autoignition of isooctane due to the increase of temperature after decomposition. 4191


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Low-Temperature Combustion Characteristics of a n-Butanol

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